With regard to such as recirculation pump control in nuclear power generation plants, gas turbine control and steam turbine control in thermal power generation plants and controls in variety of plants such as chemical plants, a stable operation thereof is demanded in the world, therefore, protective functions of particularly high importance for enhancing reliability and safety of the plant control are constituted by hard wire circuits. However, in response to cost reduction requirement the hard wire circuits are these days being constituted by soft wares, however, in, order to maintain reliability and safety, the soft wares are constituted in a triplex structure.
In a conventional triplex control system, each process input/output unit is independently connected to each of three microcomputers in a redundant structure, and the system is constituted so that the output signals are selected at the outside thereof. Further, in order to simplify the external circuit, as disclosed, for example, in JP-A-5-241606 a method of forming the output signal by summing the three signals is proposed.
Further, inputting/outputting of process variables and monitoring thereof in a multiplexed control system are performed through a separate provision of a microcomputer exclusive for the monitoring and an input/output hard interface unit. In this instance, the input/output are performed via an input/output hard interface unit exclusive for input/output and monitoring other than a control use microcomputer and information is sent to all of the control use microcomputers via transmission, thereby, standardization of information is designed.
Further, as disclosed in JP-A-08-106301, processing units and a process input/output unit are connected via triplexed lines, and a circuit which determines connection of to which unit among the units and data use of in which line among the lines is incorporated within the process input/output unit, thereby, a provision of a common process input/output unit other than the triplexed process input/output unit can be achieved to realize a cost reduction of the system.